In the current state of integrated circuit technology, transistors are typically heavily incorporated into integrated circuits in order to perform a number of functions. In an effort to improve circuit functionality, particularly for power amplifier circuits, transistors, such as heterojunction bipolar transistors (HBT) and field effect transistors (FET), for example, have been co-integrated on single gallium arsenide substrates.
Co-integrating transistor devices sometimes involves compromising between manufacturability and device performance. For some co-integrated transistor devices, for example, metal-semiconductor field effect transistor (MESFET) specific layers are grown within an emitter layer of an HBT, which while featuring manufacturability and simplicity of process, may result in degraded device performance at least in part to incomplete decoupling of the transistors.
Another exemplary co-integrated device is a pseudo-morphic high-electron mobility transistor (pHEMT) epitaxial structure co-integrated into a subcollector layer of an HBT. Although device performance may be enhanced relative to the described MESFET/HBT device, manufacturability may suffer at least in that additional processing operations may be required and epitaxial crystal growth may be more complex.